Magnetic-disk substrate and magnetic disk

ABSTRACT

A magnetic-disk substrate has an average value of squares of inclinations that is 0.0025 or less, and a frequency at which squares of inclinations are 0.004 or more is 15% or less, in a case where samples of inclinations on a main surface are obtained at intervals of 10 nm. The main surface is configured to receive at least a magnetic recording layer thereon. The magnetic-disk substrate includes an outer circumferential end portion and an inner circumferential end portion, and the outer circumferential end portion and the inner circumferential end portion have chamfered portions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.16/368,046, filed on Mar. 28, 2019, now U.S. Pat. No. 11,081,134, whichis a continuation application of U.S. patent application Ser. No.15/401,615, filed on Jan. 9, 2017, which is a continuation applicationof U.S. patent application Ser. No. 14/424,261, filed on Feb. 26, 2015,now U.S. Pat. No. 9,564,166, which is a U.S. National Stage Applicationof International Application No. PCT/JP2013/067944, filed on Jun. 28,2013, which claims priority under 35 U.S.C. § 119(e) to U.S. ProvisionalPatent Application No. 61/694,521, filed on Aug. 29, 2012, and theentire contents of U.S. patent application Ser. Nos. 14/424,261,15/401,615, and 16/368,046, International Application No.PCT/JP2013/067944, and U.S. Provisional Patent Application No.61/694,521 are hereby incorporated herein by reference.

BACKGROUND Field of the Invention

The present invention relates to a magnetic-disk glass substrate and amagnetic disk.

Background Information

Nowadays, personal computers, digital versatile disc (DVD) recorders,and the like have a built-in hard disk drive (HDD) for data recording.In particular, a magnetic disk in which a magnetic recording layer isprovided on a glass substrate is used in a hard disk drive that is usedin a device premised on portability, such as a notebook-type personalcomputer, and magnetic recording information is recorded on or read fromthe magnetic recording layer with a magnetic head that flies slightlyabove the surface of the magnetic disk. A glass substrate is unlikely tobe plastically deformed compared with a metal substrate (aluminumsubstrate) and the like, and thus is preferably used as a substrate ofthe magnetic disk.

Moreover, the density of magnetic recording has been increased to meetthe demand for an increase in the storage capacity of hard disk drives.For example, the magnetic recording information area (recording bit) hasbeen made smaller using a perpendicular magnetic recording system thatcauses the direction of magnetization in the magnetic recording layer tobe perpendicular to the surface of the substrate. A magnetic disk of theperpendicular magnetic recording system is obtained by forming anattaching layer, a soft magnetic layer (soft under layer: SUL), a baselayer, a magnetic recording layer, a protecting layer, a lubricantlayer, and the like in this order on a metal substrate or a glasssubstrate, for example. Employing the perpendicular magnetic recordingsystem makes it possible to increase the storage capacity per disksubstrate. Also, in order to further increase the storage capacity, thedistance between the recording and reproducing element and the magneticrecording layer is made very short by causing the element of themagnetic head to project farther, thus further improving the accuracy ofthe recording and reproducing of information (improve the S/N ratio). Itshould be noted that such control of the recording and reproducingelement of the magnetic head is called a dynamic flying height (DFH)control mechanism and a magnetic head equipped with this controlmechanism is called a DFH head. A magnetic-disk glass substrate that isused in an HDD in combination with such a DFH head is produced such thatthe surface roughness of the substrate is significantly small, in orderto prevent the substrate from colliding or coming into contact with themagnetic head and the recording and reproducing element that projectsfarther therefrom.

On the other hand, it is known that the surface shape of a magnetic-diskglass substrate affects the crystalline orientation dispersion (Δθ50;deviation of crystals from the perpendicular direction) of a specificcrystal face, such as a Co (002) face or an Ru (002) face, of a magneticparticle in the magnetic recording layer formed on the substrate. Δθ50is calculated as a half-value width of a peak in the case where θ/2θ ismeasured with an X-ray diffractometer, a 20 value is measured from thepeak top of the crystal face of the magnetic recording layer and a θscan is performed while fixing the 2θ. The crystalline orientationdispersion Δθ50 is an index indicating the dispersion of an axis of easymagnetization, and the smaller this value is, the better. By improvingthe Δθ50 (i.e., by bringing the Δθ50 close to zero), it is possible toobtain excellent magnetic properties and to improve the signal-to-noiseratio (SNR), and therefore, it is possible to further increase therecording density.

Regarding the crystalline orientation dispersion (Δθ50), JP 2009-140584Adescribes a magnetic-disk glass substrate that is prepared not based ona surface roughness Ra but so as to have a root-mean-square value ofinclination angles that is smaller than or equal to a predeterminedvalue (for example, 5 degrees or less, and more preferably 3 degrees orless) in order to improve the crystalline orientation dispersion (Δθ50)and the SNR of the magnetic recording layer.

Moreover, JP 2008-293552A describes a magnetic-disk glass substrate inwhich the surface roughness Ra satisfies the relationship Ra≤0.15 nm andthe average inclination angle is set to be 2 degrees or less. Using thissubstrate makes it possible to reduce Δθ50 and to reduce the mediumnoise (evaluated at a linear recording density of 825 kbpi using a TMRhead of 130 Gbpsi). It should be noted that in the case where the linearrecording density is 825 kbpi, a recording bit length (hereinafter,indicates a calculated value based on linear recording density) is about30 nm.

SUMMARY

When the inventors of the present invention produced magnetic disksusing magnetic-disk glass substrates (also referred to merely as“substrates” hereinafter) in which the surface roughness of the mainsurface of each substrate or the root-mean-square value of inclinationangles (or the average inclination angle) thereof was set to be smallerthan or equal to a predetermined value, it was found that there werecases where the SNR of reproduced signals from a magnetic disk having ahigh recording density was not necessarily improved despite the surfaceroughness or the root-mean-square value of inclination angles of themain surface being set to be sufficiently small and the Δθ50 being setto be sufficiently small. That is, it was found that as the surfaceroughness or the root-mean-square value of inclination angles of themain surface of the substrate decreased to a certain extent, the SNR ofreproduced signals from a magnetic disk produced using this substratewas improved, but that there were cases where the SNR of the reproducedsignals from the magnetic disk was not improved even when the surfaceroughness or the root-mean-square value of the main surface was furtherreduced beyond that extent.

In recent years, the amount of projection of the recording andreproducing element of the magnetic head has been increased in order toachieve a high recording density of, for example, 600 GB/P or greater,and thus the gap between the element and the magnetic disk becomessignificantly small. As a result, writing is possible at a higherrecording density than before. It is thought that magnetic disks will bemade to have still higher recording densities in future, and thatimproving the SNR of reproduced signals from a magnetic disk having ahigh recording density will become an even more important factor.

Therefore, it is an object of the present invention to provide amagnetic-disk glass substrate and a magnetic disk with which the SNR ofreproduced signals from a magnetic disk having a higher recordingdensity than before can be improved.

In order to find out the cause for not observing an improvement in theSNR of the magnetic disk having a high recording density despite thesurface roughness of the main surface or the root-mean-square value ofinclination angles (or average inclination angle) of the main surfacebeing set to be sufficiently small, the inventors of the presentinvention investigated the properties of the main surfaces of varioussubstrates in detail. As a result, it was found that there were caseswhere the SNR of reproduced signals from magnetic disks were differenteven when the magnetic disks had main surfaces whose surface roughnessesor root-mean-square values of inclination angles (or average inclinationangles) were substantially the same.

Therefore, regarding the surface shapes of various substrates, theinventors of the present invention focused on not the average value ofthe angles of inclinations on the main surface (root-mean-square valueof inclination angles or average inclination angle) but the individualinclination values. As a result, it was found that the frequency ofinclinations of minute gaps having a value that was equal to or greaterthan a predetermined value was high in the main surfaces of thesubstrates used in the magnetic disks having a high recording densitythat tended to have a relatively low SNR. It should be noted that, forexample, regarding the properties of the main surface of a substrate, inthe case where two predetermined points in a minute gap on the mainsurface are selected, the inclination is a value obtained by dividingthe amount of change in height between the two points by the length ofthe minute gap.

The inventors of the present invention think that the reason why the SNRof a magnetic disk is not improved in the case where large inclinationsexist due to the variation of the inclinations on the main surface is asfollows.

That is, if a large inclination exists on the main surface of asubstrate, it is thought that the direction of the crystallineorientation in a magnetic recording layer formed directly over theposition on the substrate where the large inclination exists inclinessignificantly from the perpendicular direction or crystals do notepitaxially grow appropriately, resulting in defects. However, it isthought that such a problem does not manifest as reproduction noise inthe case where a conventional recording bit length (e.g., about 30 nm asdescribed above) is applied. This point will be described with referenceto FIGS. 1A and 1B. FIGS. 1A and 1B show the influence of cases where aportion with a large inclination exists on the substrate when therecording bit length is long and when the recording bit length is short.In the case where the recording bit length is long as shown in FIG. 1A,even if magnetic particles that incline from the perpendicular directionor become defects exist directly over the position where there is alarge inclination on the substrate, a large number of other magneticparticles included in one recording bit length form a correct magneticfield, and therefore, it is assumed that correct signals are read out asa whole and signal quality is not affected.

On the other hand, when the recording bit length is short as shown inFIG. 1B due to a recording density being made higher, in the case wheremagnetic particles that incline from the perpendicular direction orbecome defects exist directly over the position where a largeinclination exists on the substrate, the influence of the magneticparticles is relatively increased, and thus the possibility thatreproduced signals from the area including those magnetic particles willbe incorrect (that is, cause noise) increases. That is, in the casewhere the recording bit length is short, it is assumed that the SNR of amagnetic disk is not improved due to large inclinations existing on themain surface of the substrate. It is thought that the problem of the SNRof a magnetic disk not improving first became manifest because recordingat a linear recording density of, for example, 2000 kbpi or greater(e.g., a recording bit length of about 12.70 nm or less) becamepossible. In particular, the recording bit length becomes significantlyshort in a magnetic disk having a high recording density, and therefore,the above-described problem becomes marked.

It should be noted that the inventors of the present invention comparedΔθ50 of the magnetic disks having a high recording density in which adifference in the SNRs of the reproduced signals occurred, but no markeddifference was observed. It is thought that the region on a substratehaving a large inclination that affects the SNR is insignificant as awhole and was thus not detected with an X-ray diffractometer during thecalculation of Δθ50. That is, Δθ50 is a half-value width of a peakobserved by an X-ray diffractometer and is merely an index indicatingvariation of the axes of easy magnetization, and thus it is thought thateven if half-value widths are the same as each other for example,variation of the axes of easy magnetization differs according to anextent of the spread of θ in a low X-ray intensity region.

The inventors of the present invention further think that the reason whylarge inclinations did not conventionally become manifest asreproduction noise in the case where the large inclinations existed onthe main surface of a substrate is that the total thickness of anattaching layer and an SUL (i.e., the thickness of amorphous layers) waslarge. This point will be described with reference to FIGS. 2A and 2B.FIGS. 2A and 2B show the influence of the cases where a region having alarge inclination exists on a substrate when the thickness between thesubstrate and a magnetic recording layer is large and when the thicknessis small.

The total thickness of conventional amorphous metal layers such as anattaching layer (e.g., CrTi) and an SUL (e.g., FeCoTaZr) formed bysputtering or the like is relatively large (for example, about 50 nm ormore), and therefore, even if irregular portions with a largeinclination exist on the substrate, it is thought that the formedamorphous metal films have the effect of improving the irregularportions (i.e., reducing the inclinations) on the substrate, thus makingthe crystalline orientation of the magnetic particles in the magneticrecording layer favorable (in FIG. 2A).

However, in recent years, the thickness of an SUL has been reduced to,for example, 30 nm or less in order to reduce noise due to the magnetismof the SUL itself, and thus the effect of reducing the inclinations ofthe irregular portions in the substrate has decreased (in FIG. 2B).Therefore, the above-described problem of the SNR of a magnetic disk notimproving becomes more marked.

As a result of intensive research by the inventors of the presentinvention based on the above-described observations, it was found thatthe above-described problem can be solved by limiting the frequency ofinclinations having a value that is greater than or equal to apredetermined value regarding the surface properties of the main surfaceof a magnetic-disk glass substrate, allowing the present invention to beachieved.

More specifically, a first aspect of the present invention is amagnetic-disk substrate in which an average value of squares ofinclinations is 0.0025 or less and a frequency at which squares ofinclinations are 0.004 or more is 15% or less, in a case where samplesof inclinations on a main surface are obtained at intervals of 10 nm,and the main surface is configured to receive at least a magneticrecording layer thereon. The magnetic-disk substrate includes an outercircumferential end portion and an inner circumferential end portion,and the outer circumferential end portion and the inner circumferentialend portion have chamfered portions.

A second aspect of the present invention is a magnetic disk having atleast the magnetic recording laver formed on the main surface of themagnetic-disk substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show the influence of cases where a portion having alarge inclination exists on a magnetic-disk glass substrate when arecording bit length is long and when a recording bit length is short;

FIGS. 2A and 2B show the influence of cases where a portion having alarge inclination exists on a substrate when the thickness between thesubstrate and a magnetic recording layer is large and when the thicknessis small;

FIG. 3 is a drawing illustrating a method for measuring an inclinationon a main surface of a magnetic-disk glass substrate;

FIG. 4 is an exploded perspective view of a polishing device(double-side polishing device) used in first polishing processing; and

FIG. 5 is a cross-sectional view of the polishing device (double-sidepolishing device) used in first polishing processing.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a method for manufacturing a magnetic-glass substrate ofthis embodiment will be described in detail.

Aluminosilicate glass, soda-lime glass, borosilicate glass, or the likecan be used as a material for the magnetic-disk glass substrate of thisembodiment. In particular, aluminosilicate glass can be preferably usedin that it can be chemically strengthened and a magnetic-disk glasssubstrate having excellent flatness of its main surface and excellentstrength of the substrate can be produced. It is more preferable to useamorphous aluminosilicate glass.

Although there is no limitation on the composition of the magnetic-diskglass substrate of this embodiment, the glass substrate of thisembodiment is amorphous aluminosilicate glass that preferably contains,in terms of oxide amount in mol %, SiO₂ in an amount of 50 to 75%, Al₂O₃in an amount of 1 to 15%, at least one component selected from Li₂O,Na₂O and K₂O in a total amount of 5 to 35%, at least one componentselected from MgO, CaO, SrO, BaO and ZnO in a total amount of 0 to 20%,and at least one component selected from ZrO₂, TiO₂, La₂O₃, Y₂O₃, Ta₂O₅,Nb₂O₅ and HfO₂ in a total amount of 0 to 10%.

Also, the glass substrate of this embodiment may be amorphousaluminosilicate glass that preferably contains, in mass %, SiO₂ in anamount of 57 to 75%, Al₂O₃ in an amount of 5 to 20% (it should be notedthat the total amount of SiO₂ and Al₂O₃ is 74% or more), ZrO₂, HfO₂,Nb₂O₅, Ta₂O₅, La₂O₃, Y₂O₃ and TiO₂ in a total amount of more than 0% to6% or less, Li₂O in an amount of more than 1% to 9% or less, Na₂O in anamount of 5 to 18% (it should be noted that a mass ratio Li₂O/Na₂O is0.5 or less), K₂O in an amount of 0 to 6%, MgO in an amount of 0 to 4%,CaO in an amount of more than 0% to 5% or less (it should be noted thatthe total amount of MgO and CaO is 5% or less and the content of CaO islarger than that of MgO), and SrO+BaO in an amount of 0 to 3%.

Also, the glass substrate of this embodiment may contain, in terms ofoxide amount in mass %,

SiO₂: 45.60 to 60%,

Al₂O₃: 7 to 20%,

B₂O₃: 1.00 or more to less than 8%,

P₂O₅: 0.50 to 7%,

TiO₂: 1 to 15%, and

the total amount of RO (it should be noted that R represents Zn and Mg):5 to 35%.

In this case, it is also preferable to use glass containing CaO in anamount of 3.00% or less, BaO in an amount of 4% or less, no PbO, noAs₂O₃, no Sb₂O₃, no Cl⁻ component, no⁻ component, no SO³⁻ component, andno F⁻ component (second glass composition). By performingcrystallization processing on such glass, crystallized glass can beobtained that contains one or more selected from RA₁₂O₄ and R₂TiO₄ (itshould be noted that R represents one or more selected from Zn and Mg)as a main crystal phase and in which the particle size of crystals inthe main crystal phase is within a range of 0.5 nm to 20 nm, the degreeof crystallization is 15% or less, and the specific gravity is 2.95 orless.

That is, crystallized glass may be used that contains SiO₂ in an amountof 45.60 to 60%, Al₂O₃ in an amount of 7 to 20%, B₂O₃ in an amount of1.00 to 8% or less, P₂O₅ in an amount of 0.50 to 7%, TiO₂ in an amountof 1 to 15%, and RO (it should be noted that R represents Zn and Mg) ina total amount of 5 to 35%, CaO in an amount of 3.00% or less, BaO in anamount of 4% or less, no PbO component, no As₂O₃ component, no Sb₂O₃component, no Cl⁻ component, no⁻ component, no SO²⁻ component, no F⁻component, and one or more selected from RAl₂O₄ and R₂TiO₄ (it should benoted that R represents one or more selected from Zn and Mg) as a maincrystal phase, and in which the particle size of crystals in the maincrystal phase is within a range of 0.5 nm to 20 nm, the degree ofcrystallization is 15% or less, and the specific gravity is 2.95 orless.

Also, the composition of glass used in this embodiment is, for example,

SiO₂: 35 to 65 mol %,

Al₂O₃: 5 to 25 mol %,

MgO: 10 to 40 mol %, and

TiO₂: 5 to 15 mol %.

In this case, a glass composition (first glass composition) in which thetotal amount of the above components is 92 mol % or more is preferable.By performing crystallization processing on such glass, crystallizedglass can be obtained that contains enstatite and/or a solid solutionthereof as a main crystal.

The magnetic-disk glass substrate of this embodiment is a thin glasssubstrate with an annular shape. Although there is no limitation on thesize of the magnetic-disk glass substrate, the magnetic-disk glasssubstrate is preferable, for example, as a magnetic-disk glass substratewith a nominal diameter of 2.5 inches.

On a main surface of the magnetic-disk glass substrate of thisembodiment, in the case where samples of an inclination are obtained atintervals of about 10 nm, the average value of the squares of theinclinations is 0.0025 or less and the frequency at which the squares ofthe inclinations are 0.004 or more is 15% or less. It is more preferablethat the frequency at which the squares of the inclinations are 0.004 ormore is 10% or less. By causing the main surface to have such a shape,the number of the portions in which large inclinations are formed on themain surface is significantly reduced. Therefore, there will be very fewregions where the crystalline orientation in the magnetic recordinglayer formed on the magnetic-disk glass substrate is largely inclinedfrom the perpendicular direction with respect to the main surface, andthat crystals do not grow appropriately.

It should be noted that the object of the present invention cannot beachieved only by reducing the variation of the inclinations on the mainsurface of the glass substrate. Although the frequency of the largeinclinations on the main surface tends to decrease by reducing thevariation of the inclinations (such as the dispersion of theinclinations) on the main surface, the distribution of the inclinationsthemselves does not necessarily follow a normal distribution. Therefore,even in the case where the variations of the inclinations are the same,the frequencies of inclusion of large inclinations are sometimessignificantly different. If the frequency of inclusion of largeinclinations increases, there is a possibility that magnetic signalsfrom the portions in which the large inclinations exists cause noise.Therefore, by reducing the frequency at which the squares of theinclinations are greater than or equal to a specific value in additionto the frequency at which the average value of the squares of theinclinations is smaller than or equal to a predetermined value, a glasssubstrate can be obtained that includes a small number of the largeinclinations on its main surface.

In particular, a recording bit length becomes significantly short in amagnetic disk having a high recording density. However, even in thiscase, the SNR of reproduced signals from the magnetic disk is improved.

A method for measuring inclinations on a main surface of a magnetic-diskglass substrate (referred to merely as “substrate” hereinafter) will bedescribed. Inclinations on the main surface are measured using an atomicforce microscope (AFM). Data observed with an AFM is data of a heightZ(x,y) at positions arranged at the equal intervals in a measurementplane (x-y plane) of the substrate.

For example, as shown in FIG. 3 , the height Z(x,y) at respectivepositions that divide both the x direction and the y direction of ameasurement region of 1 μm square on the main surface of the substrateinto 512 is measured, and the root-mean-square inclination Sdq iscalculated based on the values of Z(x,y) at intervals of about 10 nm(more precisely 9.76 nm) in accordance with the following equation 1.

The “average value of the squares of the inclinations” in the presentinvention is a square of the root-mean-square inclination Sdq.

$\begin{matrix}{{Sdq} = {\sqrt{\frac{1}{A}{\underset{A}{\int\int}\left\lbrack {\left( \frac{\partial{Z\left( {x,y} \right)}}{\partial x} \right)^{2} + \left( \frac{\partial{Z\left( {x,y} \right)}}{\partial y} \right)^{2}} \right\rbrack}}dxdy}} & \left( {{Equation}1} \right)\end{matrix}$

On the other hand, the inclination dq on the main surface at a minutegap (about 10 nm) between the neighboring sampling positions isrepresented by the following equation 2.

The “square of the inclination” in the present invention is a square ofthe inclination dq.

$\begin{matrix}{{dq} = \sqrt{\left\lbrack {\left( \frac{\partial{Z\left( {x,y} \right)}}{\partial x} \right)^{2} + \left( \frac{\partial{Z\left( {x,y} \right)}}{\partial y} \right)^{2}} \right\rbrack}} & \left( {{Equation}2} \right)\end{matrix}$

It should be noted that it is preferable that the distance between theneighboring sampling positions is close to the recording bit length of amagnetic disk having a high recording density. Measuring inclinations atresolution close to the recording bit length makes it easy to correlatea measurement value with the crystalline orientation of the magneticparticles included in one recording bit of a magnetic recording layer,and in turn, with the SNR of reproduced signals. For example, it ispreferable that the length of the gap between the sampling positions isset to be about half to two times the recording bit length to be assumed(for example, about 12.70 nm or less at a recording density of 2000 kbpior greater).

In the present invention, for example, “the frequency at which thesquares of the inclinations are 0.004 or more is 15% or less” means thatthe ratio (or proportion) of the number of samples in which the squaresof the inclinations are 0.004 or more to the number of samples for thesquares of the inclinations obtained in the main surface is 15% or less.

In the case where sampling is performed at intervals of about 10 nm, theaverage value of the squares of the inclinations is 0.0025 or less andthe frequency at which the squares of the inclinations are 0.004 or moreis 15% or less in the magnetic-disk glass substrate of this embodiment.Therefore, when a magnetic disk is produced using this glass substrate,the SNR of the magnetic disk is improved.

It should be noted that it is preferable to use amorphous glass for themagnetic-disk glass substrate of this embodiment as described abovebecause it becomes possible to smooth the main surface with highaccuracy and to easily reduce the frequency of inclinations having alarge angle. Moreover, by chemically strengthening the above-describedamorphous glass, it is possible to form a compressive stress layer onthe surface layer and to enhance the impact resistance of themagnetic-disk glass substrate.

Crystallized glass (crystal glass) may be used for the magnetic-diskglass substrate of this embodiment. Using crystallized glass makes itpossible to enhance the hardness of the glass substrate and to enhancethe impact resistance thereof.

[Method for Manufacturing Magnetic-Disk Glass Substrate]

Hereinafter, a method for manufacturing a magnetic-disk glass substrateof this embodiment will be described for each processing. It should benoted that the processing order may be changed as appropriate.

(1) Raw Glass Plate Molding and Rough Grinding Processing

After forming a sheet of plate glass by, for example, a float method, araw glass plate having a predetermined shape from which a magnetic-diskglass substrate is to be made is cut out from this sheet of plate glass.A raw glass plate may also be molded by, for example, pressing using anupper mold and a lower mold instead of a float method. It should benoted that a method for manufacturing a raw glass plate is not limitedto these methods and a raw glass plate can also be manufactured by aknown manufacturing method such as a down draw method, a redraw methodor a fusion method.

It should be noted that rough grinding processing using free abrasivegrains may be performed on two main surfaces of the raw glass plate asneeded.

(2) Inner Hole Forming Processing

An inner hole is formed in the center of the raw glass plate using, forexample, a cylindrical drill, and thus an annular raw glass plate isobtained.

(3) Shaping Processing

After the inner hole forming processing, shaping processing in whichchamfered portions are formed at the end portions (outer circumferentialend portion and inner circumferential end portion) is performed. In theshaping processing, the outer circumferential end portion and the innercircumferential end portion of the annular raw glass plate are chamferedwith a grindstone using diamond abrasive grains, or the like, and thusthe chamfered portions are formed. Also, the outer diameter and theinner diameter may be adjusted simultaneously at this time.

(4) End Surface Polishing Processing

Next, the end surfaces of the annular raw glass plate are polished.

In the end surface polishing, the side wall surface (end surface) on theinner circumference side of the raw glass plate and the side wallsurface (end surface) on the outer circumference side thereof are givena mirror finish by performing brushing. In this case, a slurrycontaining fine grains of cerium oxide, zirconium oxide or the like asfree abrasive grains is used. By polishing the end surfaces,contamination by attached waste and the like, and damage or impairmentsuch as scratches on the side wall surfaces of the raw glass plate areeliminated, and therefore, it is possible to prevent thermal asperityand the deposition of ions such as sodium and potassium that causescorrosion.

(5) Precision Grinding Processing

In precision grinding processing, the main surfaces of the annular rawglass plate are ground using a double-side grinding device provided witha planetary gear mechanism. The machining allowance in grinding is, forexample, about several micrometers to 100 micrometers. The double-sidegrinding device has a pair of upper and lower surface plates (uppersurface plate and lower surface plate) and the annular raw glass plateis held between the upper surface plate and the lower surface plate. Theraw glass plate and the surface plates are relatively moved by movingone or both of the upper surface plate and the lower surface plate, sothat two main surfaces of the raw glass plate can be ground. A surfaceplate in which fixed abrasive grains obtained by fixing abrasive grainsmade of diamond or the like with a resin are stuck on its surface can beused as the surface plate.

(6) First Polishing (Main Surface Polishing) Processing

Next, the first polishing is performed on the ground main surfaces ofthe raw glass plate. The machining allowance in the first polishing is,for example, about several micrometers to 50 micrometers. The firstpolishing is performed in order to eliminate scratches and distortionthat remain on the main surfaces due to grinding with fixed abrasivegrains and to adjust waviness and minute waviness. For example, ceriumoxide abrasive grains or zirconia abrasive grains (grain size: diameterof about 1 to 2 μm) that are suspended in a slurry are used as the freeabrasive grains for the first polishing.

(6-1) Polishing Device

A polishing device used in the first polishing processing will bedescribed with reference to FIGS. 4 and 5 . FIG. 4 is an explodedperspective view of the polishing device (double-side polishing device)used in the first polishing processing. FIG. 5 is a cross-sectional viewof the polishing device (double-side polishing device) used in the firstpolishing processing. It should be noted that a configuration similar tothis polishing device can also be applied in the grinding device used inthe grinding processing described above.

As shown in FIG. 4 , the polishing device has a pair of upper and lowersurface plates, that is, an upper surface plate 40 and a lower surfaceplate 50. An annular raw glass plate G is held between the upper surfaceplate 40 and the lower surface plate 50, and the raw glass plate G andthe surface plates are relatively moved by moving one or both of theupper surface plate 40 and the lower surface plate 50, so that two mainsurfaces of the raw glass plate G can be polished.

The configuration of the polishing device will be more specificallydescribed with reference to FIGS. 4 and 5 .

In the polishing device, planar polishing pads 10 having an annularshape as a whole are attached to the upper surface of the lower surfaceplate 50 and the bottom surface of the upper surface plate 40. A carrier30 has a toothing 31 that is provided on its outer circumferentialportion and is meshed with a sun gear 61 and an inner gear 62, and oneor more hole portions 32 for accommodating and holding the raw glassplate G. The sun gear 61, the inner gear 62 provided on the outer edgeand the disc-shaped carrier 30 constitute, as a whole, a planetary gearmechanism that has a central axis CTR as the center. The disc-shapedcarrier 30 is meshed with the sun gear 61 on the inner circumferentialside and with the inner gear 62 on the outer circumferential side, andaccommodates and holds one or more raw glass plates G (workpieces). Onthe lower surface plate 50, the carrier 30 revolves around the sun gearwhile rotating on its own axis as a planetary gear, and the raw glassplate G and the lower surface plate 50 are moved relative to each other.For example, when the sun gear 61 rotates in a counterclockwise (CCW)direction, the carrier 30 rotates in a clockwise (CW) direction and theinner gear 62 rotates in the CCW direction. As a result, the polishingpad 10 and the raw glass plate G are moved relative to each other. Theraw glass plate G and the upper surface plate 40 may be moved relativeto each other in the same manner.

During the relative motion described above, the upper surface plate 40is pressed against the raw glass plate G (that is, in a verticaldirection) with a predetermined load, and the polishing pad 10 ispressed against the raw glass plate G. Moreover, a polishing liquid(slurry) is supplied between the raw glass plate G and the polishing pad10 from a polishing liquid supplying tank 71 via one or more pipes 72with a pump (not shown). The main surfaces of the raw glass plate G arepolished by a polishing material contained in this polishing liquid.

It should be noted that although any material can be used for thepolishing pad, it is preferable to use a polishing pad with a resinpolisher made of polyurethane. It should be noted that in this polishingdevice, it is preferable that the load from the upper surface plate 40that is applied to the raw glass plate G is adjusted in order to set adesired polishing load with respect to the raw glass plate G. The loadis preferably 50 g/cm² or more, more preferably 70 g/cm² or more, andeven more preferably 90 g/cm² or more, for the purpose of achieving ahigh polishing speed. Moreover, the polishing load is preferably 180g/cm² or less, more preferably 160 g/cm² or less, and even morepreferably 140 g/cm² or less, for the purpose of reducing scratches andstabilizing quality. That is, the load is preferably 50 to 180 g/cm²,more preferably 70 to 160 g/cm², and even more preferably 90 to 140g/cm².

The supplying speed of the polishing liquid during the polishingprocessing is varied in accordance with the polishing pad 10, thecomposition and concentration of the polishing liquid and the size ofthe raw glass plate G, but is preferably 500 to 5000 ml/minute, morepreferably 1000 to 4500 ml/minute, and even more preferably 1500 to 4000ml/minute, for the purpose of enhancing the polishing speed. Therotation rate of the polishing pad 10 is preferably 10 to 50 rpm, morepreferably 20 to 40 rpm, and even more preferably 25 to 35 rpm.

In the first polishing processing, polishing is performed such that themain surface of the raw glass plate has a roughness (Ra) of 0.5 nm orless and a micro-waviness (MW-Rq) of 0.5 nm or less in terms of thesurface roughness. Here, the micro-waviness can be expressed as an RMS(Rq) value that is calculated as the roughness in a wavelength bandwidthof 100 to 500 μm in a main surface region, and can be measured with anoptical surface shape measurement device, for example.

The roughness of the main surface is expressed as the arithmetic averageroughness Ra defined by JIS B0601: 2001, and can be measured with anAFM, for example. Herein, the arithmetic average roughness Ra measuredwith a resolution of 512 by 512 pixels in a measurement area of 1 μmsquare can be used.

(7) Chemical Strengthening Processing

Next, the raw glass plate on which the first polishing has beenperformed is chemically strengthened. For example, a mixed solution ofpotassium nitrate and sodium sulfate can be used as a chemicalstrengthening liquid.

In this manner, by immersing the raw glass plate in the chemicalstrengthening liquid, lithium ions and sodium ions in the surface layerof the raw glass plate are respectively exchanged with sodium ions andpotassium ions with a relatively large ion radius in the chemicalstrengthening liquid, and the raw glass plate is strengthened.

(8) Second Polishing Processing

Next, final polishing is performed, using the same double-side polishingdevice as used in the first polishing, on the raw glass plate that hasbeen chemically strengthened and sufficiently cleaned. In this case, apolishing pad with a soft polisher (suede) (for example, polyurethanefoam with an Asker C hardness of 75) as a resin polisher is used. It ispreferable that the hardness of the polishing pad is set to be within arange of 60 to 90 in an Asker C hardness. This second polishingprocessing is mirror-polishing processing to finish the main surfaces ofthe glass substrate into smooth mirror surfaces with a surface roughnessof 0.15 nm or less in Ra while retaining the flat surfaces obtained inthe first polishing processing described above. RO water in whichcolloidal silica (average grain diameter (D50): 10 to 50 nm) isdispersed and a predetermined amount of aluminum sulfate is added anddissolved is used as the polishing liquid. It is preferable that theconcentration of aluminum sulfate is set to 0.001 to 1 mol/L. Here, itshould be noted that the average grain diameter (D50) denotes a graindiameter at which a cumulative curve reaches 50% when the cumulativecurve is found by setting the entire volume of powder particles in thegrain diameter distribution measured by a light scattering method to100%, and is a value obtained by measuring grain diameters using, forexample, a grain diameter/grain size distribution measurement device.

Subsequently, rinsing processing is performed using the same double-sidepolishing device as is. In the rinsing processing, for example, RO waterin which an appropriate amount of aluminum sulfate has been added anddissolved is used as a processing liquid to be supplied between thepolishing pad and the glass substrate. It is sufficient that theconcentration of aluminum sulfate is the same as described above.

The glass substrate on which the rinsing processing has been performedis immersed in a cleaning bath, subjected to ultrasonic cleaning, andthen dried, and thus a magnetic-disk glass substrate is obtained. Itshould be noted that if the glass substrate is cleaned using an acidiccleaning liquid, the main surface becomes rough and the variation ofinclinations on the main surface increases, and therefore, it ispreferable to use a neutral cleaning liquid or an alkaline cleaningliquid. It should be noted that when using a neutral cleaning liquid oran alkaline cleaning liquid, it is preferable that the pH of thecleaning liquid is set to be within a range of 6 to 11. Also, it ispreferable not to use an acidic cleaning liquid whose pH is 5 or less.

A magnetic-disk glass substrate in which the variation of inclinationson the main surface is small and the frequency of large inclinations issmall is obtained through the second polishing processing, rinsingprocessing and cleaning. That is, in the case where samples of aninclination are obtained at intervals of about 10 nm, a magnetic-glasssubstrate in which the average value of the squares of the inclinationsis 0.0025 or less and the frequency at which the squares of theinclinations are 0.004 or more is 15% or less is obtained. It should benoted that the magnitude of the inclination on the main surface can beadjusted as appropriate by the polishing time, rinsing time and cleaningtime.

The method for manufacturing a magnetic-disk glass substrate of thisembodiment has been described above for each processing, but theprocessing order is not limited to the above-described order. Moreover,chemical strengthening process may be omitted depending on anapplication or a glass composition of the magnetic-disk glass substrate.

[Magnetic Disk]

A magnetic disk is a magnetic recording medium having a magneticrecording layer on the magnetic-disk glass substrate of this embodiment.A magnetic disk has a configuration in which, for example, at least anattaching layer, a base layer, a magnetic recording layer, a protectinglayer and a lubricant layer are laminated on the main surface of themagnetic-disk glass substrate (referred to as “substrate” as appropriatehereinafter) in this order from the side of the main surface.

For example, the substrate is introduced into a film deposition devicethat has been evacuated and the layers from the attaching layer to themagnetic recording layer are sequentially formed on the main surface ofthe substrate in an Ar atmosphere by a DC magnetron sputtering method.For example, CrTi can be used in the attaching layer and CrRu can beused in the base layer. A CoPt based alloy can be used in the magneticrecording layer. After the film deposition as described above, byforming the protecting layer using C₂H₄ by, for example, a CVD methodand performing nitriding processing that introduces nitrogen to thesurface in the same chamber, a magnetic recording medium can be formed.Thereafter, by coating the protecting layer with perfluoropolyether(PFPE) by a dip coat method, the lubricant layer can be formed.

In addition, a soft under layer (SUL), a seed layer, an intermediatelayer and the like may be formed between the attaching layer and themagnetic recording layer by a known film deposition method such as asputtering method (including a DC magnetron sputtering method, RFmagnetron sputtering method, and the like) or a vapor deposition method.It is possible to refer to, for example, JP 2009-110626A, paragraphs[0027] to [0032] for detailed description on the above-described layers.

It should be noted that as described above, using the magnetic-diskglass substrate of this embodiment makes it possible to obtain a highSNR even if the thickness of the SUL is 30 nm or less. It should benoted that the composition of the magnetic-disk glass substrate of thisembodiment may include SiO₂, Li₂O and Na₂O, and one or more alkalineearth metal oxides selected from the group consisting of MgO, CaO, SrOand BaO as essential components, the molar ratio of the content of CaOto the total content of MgO, CaO, SrO and BaO (CaO/(MgO+CaO+SrO+BaO))may be 0.20 or less, and the glass-transition temperature may be 650° C.or higher. The magnetic-disk glass substrate having such a compositionis preferably for a magnetic-disk glass substrate to be used in amagnetic disk for energy-assisted magnetic recording.

It is thought that it is preferable to use a high Ku magnetic materialto form a magnetic recording layer in a magnetic disk forenergy-assisted magnetic recording. For example, the magnetic recordinglayer can be made of an L10 ordered alloy including a magnetic materialcontaining an alloy of Pt, and Fe and/or Co as a main component. Inorder to obtain such a magnetic recording layer, the magnetic materialcontaining an alloy of Pt, and Fe and/or Co as a main component isdeposited on the main surface of the substrate, and then annealingprocessing for ordering the layer is performed. Here, the aboveannealing processing is generally performed at a high temperature ofhigher than 500° C. Accordingly, if glass constituting the substrate haspoor heat resistance, the glass is deformed at a high temperature andflatness is impaired. In contrast, the substrate having theabove-described composition exhibits superior heat resistance(glass-transition temperature of 650° C. or higher), and therefore, highflatness can be retained after annealing processing.

It should be noted that crystallized glass may be used for themagnetic-disk glass substrate of this embodiment.

In the case where crystallized glass is formed by crystallizingprocessing, it is sufficient to perform the following processing.Specifically, for example, a plurality of glass substrates sandwichingdisc-shaped plates called a setter between each of them are introducedinto a heating furnace and subjected to heating processing. The setterscan be made of ceramics. In the heating processing, the glass substrateis crystallized by, for example, being held at a nucleus formingtemperature for a predetermined period of time and then being held at acrystal growing temperature for a predetermined period of time. Thetemperature and the period of time for nucleus formation and crystalgrowth are set as appropriate depending on the glass composition of theglass substrate. When the glass substrate is cooled after being heated,it is preferable to adjust a cooling speed during slow cooling such thatdistortion and bending do not occur in the glass substrate.

It is possible to determine whether or not the glass substrate subjectedto crystallizing processing is crystallized using, for example, adiffraction intensity distribution obtained by powder X-raydiffractometry. It should be noted that it is preferable to precipitatecrystals whose average particle size in the crystal phase is 10 nm orless for the purpose of reducing the surface roughness of the mainsurface of the glass substrate. Glass that has been crystallized(referred to as “crystallized glass” hereinafter) is a materialconfigured to have crystals precipitated inside the glass by heatingamorphous glass, and can be distinguished from amorphous glass.

In this embodiment, the Young's modulus of the glass substrate on whichthe crystallizing processing has been performed is preferably 100 GPa ormore, and more preferably 120 GPa or more. Thus, it is possible toobtain a glass substrate with high flexural strength and high impactresistance. The flexural strength of the glass substrate on which thecrystallizing processing has been performed is preferably 7 kgf or more,and particularly preferably 8 kgf or more for the purpose of enhancingthe impact resistance. Thus, it is possible to obtain a magnetic-diskglass substrate that is preferably for a hard disk drive (HDD) with ahigh rotation rate of 10000 rpm or more.

Working Examples and Comparative Examples

In order to confirm the effect of the method for manufacturing amagnetic-disk glass substrate according to this embodiment, a 2.5-inchmagnetic disk was produced from the manufactured glass substrate. Theglass composition 1 of the produced magnetic-disk glass substrate is asfollows.

(Glass Composition 1)

Amorphous aluminosilicate glass was used that contained, in mass %, SiO₂in an amount of 65.08%, Al₂O₃ in an amount of 15.14%, Li₂O in an amountof 3.61%, Na₂O in an amount of 10.68%, K₂O in an amount of 0.35%, MgO inan amount of 0.99%, CaO in an amount of 2.07%, ZrO₂ in an amount of1.98%, and Fe₂O₃ in an amount of 0.10%, and that had a glass-transitiontemperature of 510° C.

Production of Magnetic-Disk Glass Substrate of Working Examples andComparative Examples

Each type of processing of the method for manufacturing a magnetic-diskglass substrate according to this embodiment was performed in the givenorder. Here, the pressing method was used in molding of the raw glassplates in step (1). In the rough grinding processing, alumina-based freeabrasive grains were used.

In the end surface polishing in step (4), cerium oxide was used as thefree abrasive grains, and polishing was performed using a polishingbrush. In the precision grinding in step (5), grinding was performedusing a grinding device in which fixed abrasive grains obtained byfixing diamond abrasive grains with resin bond were attached to thesurface of a surface plate.

In the first polishing in step (6), polishing was performed using thepolishing device shown in FIGS. 4 and 5 for 60 minutes. Cerium oxideabrasive grains having an average grain diameter of 1 μm were used, anda hard urethane pad was used as the polishing pad.

In the chemical strengthening in step (7), a mixed solution of potassiumnitrate (60 wt %) and sodium nitrate (40 wt %), or the like was used asa chemical strengthening liquid, the chemical strengthening liquid washeated to 350° C., and the raw glass plate that had been preheated to200° C. in advance was immersed in the chemical strengthening liquid for4 hours.

In the second polishing in step (8), polishing was performed usinganother polishing device similar to the polishing device shown in FIGS.4 and 5 . A polishing pad with a soft polisher (suede) (polyurethanefoam with an Asker C hardness of 75) was used. RO water in whichcolloidal silica (average grain diameter (D50): 30 nm) was dispersed andaluminum sulfate was added and dissolved in an amount of 0.01 mol/L wasused as the polishing liquid. A load of 100 g/cm² was applied, and themachining allowance for polishing was 5 m. Subsequently, the rinsingprocessing was performed using the same polishing device as is. In therinsing processing, RO water in which aluminum sulfate was added anddissolved in an amount of 0.01 mol/L was used as a processing liquid tobe supplied between the polishing pad and the glass substrate. A load of100 g/cm² was applied, and the processing time was set to 5 minutes. Theglass substrate on which the above-described rinsing processing had beenperformed was immersed in respective cleaning baths filled with aneutral detergent, an alkaline cleaning liquid (pH: 10 or less), purewater, and IPA so as to be subjected to ultrasonic cleaning, and thenwas dried with IPA (steam drying). Thus, a magnetic-disk glass substratewas obtained. The produced magnetic-disk glass substrate was used as asubstrate for a magnetic disk with a nominal diameter of 2.5 inches(having an inner diameter of 20 mm, an outer diameter of 65 mm, and athickness of 0.635 mm).

Inclinations of the main surface were adjusted by the polishing time andcleaning time in the second polishing, and thus the magnetic-disksubstrates of working examples and comparative examples shown in Table 1were obtained. The properties of the main surface of the producedmagnetic-disk glass substrate were measured using an AFM. As describedin association with FIG. 3 , the height at respective positions thatdivide both the x direction and the y direction of a measurement regionof 1 μm square on the main surface of the produced magnetic-disk glasssubstrate into 512 was measured, and the average value of the squares ofthe inclinations at intervals of about 10 nm (precisely, 1000/512*5=9.76nm) and the frequency at which the squares of the inclinations were0.004 or more were calculated. It should be noted that the measurementpositions were 22 mm apart from the center on the main surface, and theaverage value of two measurement results was calculated.

As a result, the magnetic-disk glass substrates in which the averagevalue of the squares of the inclinations was more than 0.0025, and themagnetic-disk glass substrates in which the squares of the inclinationswas 0.0025 or less and the frequency at which the squares of theinclinations were 0.004 or more was greater than 15% were used as thecomparative examples (Comparative Examples 1 and 2 shown in Table 1),and the magnetic-disk glass substrates in which the squares of theinclinations was 0.0025 or less and the frequency at which the squaresof the inclinations were 0.004 or more was 15% or less were used as theworking examples (Working Examples 1 and 2 shown in Table 1).

It should be noted that when the arithmetic average roughness Ra of themagnetic-disk glass substrate of each working example was measured witha resolution of 512 by 512 pixels in a measurement area of 1 μm squareusing an AFM, all of the Ra values were 0.15 nm or less. [Production ofmagnetic disks of working examples and comparative examples]

Next, magnetic-disks were produced using the magnetic-disk glasssubstrates of the working examples and comparative examples.

It should be noted that film deposition was performed on themagnetic-disk glass substrate as follows. First, an attachinglayer/SUL/pre-base layer/base layer/magnetic recording layer/auxiliaryrecording layer/protecting layer/lubricant layer were sequentiallyformed on the substrate in an Ar atmosphere by a DC magnetron sputteringmethod using a film deposition device that had been evacuated. It shouldbe noted that the film deposition was performed at an Ar gas pressure of0.6 Pa unless otherwise stated. As the attaching layer, a Cr-50Ti layerwith a thickness of 4 nm was formed. As the SUL, a Ru layer with athickness of 0.7 nm and 92Co-3Ta—Zr layers with a thickness of 10 nmthat sandwiched the Ru layer were formed. As the pre-base layer, aNi-10W layer with a thickness of 8 nm was formed. As the base layer, aRu layer with a thickness of 10 nm was formed at 0.6 Pa and then a Rulayer with a thickness of 10 nm was formed thereon at 5 Pa. As themagnetic recording layer, a 90(72Co-10Cr-18Pt)-5(SiO₂)-5(TiO₂) layerwith a thickness of 15 nm was formed at 3 Pa. As the auxiliary recordinglayer, a 62Co-18Cr-15Pt-5B layer with a thickness of 6 nm was formed. Asthe protecting layer, a layer with a thickness of 4 nm was formed usingC₂H₄ by a CVD method, and the surface layer was subjected to nitridingprocessing. As the lubricant layer, a layer with a thickness of 1 nm wasformed using perfluoropolyether (PFPE) by a dip coat method.

Magnetic disks corresponding to the respective magnetic-disk glasssubstrates of the comparative examples and working examples wereobtained through the above-described manufacturing steps.

When the grain size (diameter) of a magnetic particle in the magneticrecording layer was examined by a plane observation using a TEM, theaverage value was 8 nm. Moreover, when a film-formation state waschecked by a cross-sectional observation using a TEM, the CrTi layer andSUL were in the amorphous state and the particle boundaries were notobserved, whereas the particle boundaries were observed in the NiW layerto the auxiliary recording layer and it was observed that crystals grewto have a columnar shape. [Evaluation of magnetic disks of workingexamples and comparative examples]

The signal-to-noise ratio (SNR) of reproduced signals from each producedmagnetic disk was measured under the following conditions. It should benoted that a DFH mechanism-mounting head was used. A distance from thefront end of a recording and reproducing element to the surface of themagnetic disk was set to be 1 nm by a DFH mechanism.

-   -   Linear recording density during writing signals: 2000 kbpi    -   Rotation rate of magnetic disk: 5400 rpm

It should be noted that the evaluation standard of an SNR was as followswhen the SNR of Comparative Example 1 was given as Ref (reference).Magnetic disks evaluated as “Fair”, “Good” and “Excellent” wereacceptable.

Excellent: Ref+0.5 [dB]≤SNR

Good: Ref+0.3 [dB]≤SNR

Fair: Ref<SNR<Ref+0.3 [dB]

Poor: equal to or less than Ref

TABLE 1 Average Frequency at value of which squares of squares ofinclinations are inclinations 0.004 or more SNR Comp. Ex. 1 0.0027 15%Poor Comp. Ex. 2 0.0024 19% Poor Work. Ex. 1 0.0025 15% Fair Work. Ex. 20.0019 10% Good Comp. Ex. 3 0.0022 17% Poor Work. Ex. 3 0.0019 12% FairWork. Ex. 4 0.0015  8% Good Work. Ex. 5 0.0012  5% Excellent

It is found from the results shown in Table 1 that the working examplesin which the average value of the squares of the inclinations was 0.0025or less and the frequency at which the square numbers of theinclinations were 0.004 or more was 15% or less provided a favorable SNRfor a magnetic disk. It is found that the working examples in which thefrequency at which the square numbers of the inclinations were 0.004 ormore was 10% or less provided a more favorable SNR for a magnetic disk.This is because the inclinations on the magnetic-disk glass substratesof the working examples are generally small and the frequency ofinclusion of the large inclinations is small, and thus there are veryfew regions in which the crystalline orientation in the magneticrecording layer formed on the magnetic-disk glass substrate of theworking examples is largely inclined from the perpendicular directionwith respect to the main surface and crystals do not grow appropriately.

[Evaluation with Different Glass Composition]

Next, in order to further confirm the effect of the method formanufacturing a magnetic-disk glass substrate according to thisembodiment, a 2.5-inch magnetic disk was produced from a magnetic-diskglass substrate that had a composition 2 different from theabove-described composition 1. The method for producing a magnetic-diskglass substrate is the same as the case where glass has the composition1 (that is, the above-described steps (1) to (8)). The glass composition2 is as follows. It should be noted that the glass composition 2 ispreferably for a composition of glass to be used in a magnetic-diskglass substrate to be used in a magnetic disk for energy-assistedmagnetic recording.

(Glass Composition 2)

Amorphous aluminosilicate glass was used that contained SiO₂ in anamount of 65 mol % (64.7 mass %), Al₂O₃ in an amount of 6 mol % (10.13mass %), Li₂O in an amount of 1 mol % (0.5 mass %), Na₂O in an amount of9 mol % (9.24 mass %), MgO in an amount of 17 mol % (11.35 mass %), CaOin an amount of 0 mol % (0 mass %), SrO in an amount of 0 mol % (0 mass%), BaO in an amount of 0 mol % (0 mass %), and ZrO₂ in an amount of 2mol % (4.08 mass %). It should be noted that the molar ratio of thecontent of CaO to the total content of MgO, CaO, SrO and BaO(CaO/(MgO+CaO+SrO+BaO)) was 0, and the glass-transition temperature was671° C.

A magnetic-disk glass substrate was produced using glass having theabove-described glass composition 2 under the condition that thepolishing time in the second polishing processing was the same as inWorking Example 1 (Working Example 6). As a result, as Working Example1, the average value of the squares of the inclinations was 0.0025 orless and the frequency at which the squares of the inclinations were0.004 or more was 15% or less. Accordingly, in the case where a magneticdisk is produced using the magnetic-disk glass substrate of WorkingExample 6, as in Working Example 1, it is thought that there will be fewregions in which the crystalline orientation in the magnetic recordinglayer is largely inclined from the perpendicular direction with respectto the main surface and crystals do not grow appropriately, and it canbe expected to obtain the same favorable SNR as that of Working Example1.

While the magnetic-disk glass substrate and magnetic disk according tothe present invention has been described in detail, the presentinvention is not limited to the above-described embodiment, and it willbe appreciated that various improvements and modifications can be madewithout departing from the concept of the present invention.

According to a first aspect of the embodiment, a magnetic-disk glasssubstrate in which an average value of squares of inclinations is 0.0025or less and a frequency at which squares of inclinations are 0.004 ormore is 15% or less, in a case where samples of inclinations on a mainsurface are obtained at intervals of 10 nm is provided.

In the magnetic-disk glass substrate, it is preferable that thefrequency at which squares of inclinations are 0.004 or more is 10% orless. It is preferable that the magnetic-disk glass substrate is made ofamorphous glass. It is preferable that the magnetic-disk glass substrateis made of glass having a glass-transition temperature of 650° C. ormore. In the magnetic-disk glass substrate, it is preferable that anarithmetic average roughness Ra of the main surface is 0.15 nm or less.

According to a second aspect of the embodiment, a magnetic disk havingat least a magnetic recording layer formed on the surface of theabove-described magnetic-disk glass substrate is provided.

According to a third aspect of the embodiment, a magnetic-disk glasssubstrate has a pair of flat main surfaces, wherein, after magneticrecording layers are formed above the main surfaces to produce amagnetic disk, a maximum value of an average value of squares ofinclinations and a maximum value of a frequency at which square numbersof inclinations are 0.004 or more in a case where samples ofinclinations on the main surfaces are obtained at intervals of 10 nm aredetermined such that a signal-to-noise ratio of reproduced signals whenthe signals are written in the magnetic recording layer at a linearrecording density of 2000 kbpi or greater has a favorable level.

With a magnetic-disk glass substrate and a magnetic disk according tothe present invention, it is possible to improve the SNR of reproducedsignals from a magnetic disk having a high recording density.

What is claimed is:
 1. A magnetic-disk substrate in which an averagevalue of squares of inclinations is 0.0025 or less and a frequency atwhich squares of inclinations are 0.004 or more is 15% or less, in acase where samples of inclinations on a main surface are obtained atintervals of 10 nm, the main surface is configured to receive at least amagnetic recording layer thereon, the magnetic-disk substrate includesan outer circumferential end portion and an inner circumferential endportion, and the magnetic-disk substrate is configured to constitute amagnetic disk of a perpendicular magnetic recording system.
 2. Themagnetic-disk substrate according to claim 1, wherein the frequency atwhich squares of inclinations are 0.004 or more is 10% or less.
 3. Themagnetic-disk substrate according to claim 1, wherein an arithmeticaverage roughness of the main surface is 0.15 nm or less.
 4. Themagnetic-disk substrate according to claim 1, wherein an RMS (Rq) valuethat is calculated as a roughness in a wavelength bandwidth of 100 μm to500 μm on the main surface is 0.5 nm or less.
 5. The magnetic-disksubstrate according to claim 1, wherein the magnetic-disk substrate isconfigured as a substrate of a magnetic disk for energy-assistedmagnetic recording.
 6. The magnetic-disk substrate according to claim 1,wherein the magnetic-disk substrate is made of glass.
 7. Themagnetic-disk substrate according to claim 6, wherein the glass has aglass-transition temperature of 650° C. or more.
 8. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 1. 9. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 2. 10. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 3. 11. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 4. 12. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 5. 13. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 6. 14. A magnetic diskhaving at least the magnetic recording layer formed on the main surfaceof the magnetic-disk substrate according to claim
 7. 15. The magneticdisk according to claim 8, wherein the magnetic disk has a linearrecording density of at least 2000 kbpi.
 16. The magnetic disk accordingto claim 8, wherein the magnetic disk further includes a soft magneticlayer formed on the main surface of the magnetic-disk substrate, and thesoft magnetic layer has a thickness of 30 nm or less.
 17. The magneticdisk according to claim 9, wherein the magnetic disk further includes asoft magnetic layer formed on the main surface of the magnetic-disksubstrate, and the soft magnetic layer has a thickness of 30 nm or less.18. The magnetic disk according to claim 10, wherein the magnetic diskfurther includes a soft magnetic layer formed on the main surface of themagnetic-disk substrate, and the soft magnetic layer has a thickness of30 nm or less.
 19. The magnetic disk according to claim 11, wherein themagnetic disk further includes a soft magnetic layer formed on the mainsurface of the magnetic-disk substrate, and the soft magnetic layer hasa thickness of 30 nm or less.
 20. The magnetic disk according to claim12, wherein the magnetic disk further includes a soft magnetic layerformed on the main surface of the magnetic-disk substrate, and the softmagnetic layer has a thickness of 30 nm or less.